Bit for drilling wells and associated drilling method

ABSTRACT

A bit for drilling wells has a front face with radial blades having cutting elements distributed around the front face. A space for forming a core is situated at the center of the front face. A cavity is provided for evacuating the core towards a periphery of the bit. At least a portion of the cavity is situated between adjacent blades. The cavity is delimited by two lateral surfaces and a clearance surface set back with respect to the front face, and the cavity is open in a direction opposite the clearance surface. The bit may be used in methods for drilling wells and makes it possible to rapidly drill wells of great depth in all types of rock without the risk of clogging.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/596,995, filed Oct. 22, 2009, which is the National Stage under 35USC §371 of PCT/IB2008/002299, filed Apr. 22, 2008, which claimspriority to French Patent Application No. 07/02919, filed Apr. 23, 2007.Each of the foregoing applications is incorporated herein by referencein its entirety as if set forth in full.

TECHNICAL FIELD

The present invention relates to a bit for drilling wells, and inparticular cylindrical wells of great depth such as mine shafts, oil orgas wells. The present invention also relates to a method for drillingwells using the bit.

BACKGROUND

The bit is a drilling tool intended to be installed at the end of adrill pipe string for drilling wells into a reservoir. By reservoir, ismeant rock which is sufficiently porous and permeable to be able tocontain fluids (water, oil, gas). These fluids can optionally accumulateto form a deposit.

A drill pipe string is supported by a metal derrick and is rotated by arotary table. The drilling mud, a specific mixture of clay, water andchemical products is continuously injected into the inside of the drillpipes then emerges via the bit and returns to the surface via annularspace contained between the drill pipes and the walls of the well. Thecirculation of the drilling mud cools the bit and allows the cuttings tobe evacuated. At the surface, the drilling mud is filtered andreinjected. Analysis of the cuttings provides invaluable information onthe nature and composition of the rocks cut through.

Thirty years ago, the drilling of a well of great depth could takeseveral months. Also, in order to accelerate the drilling speed, bitshave been proposed that do not drill at the centre of wells. In fact,the circumferential linear speed of the bit decreases from the peripheryof the bit towards its centre and is zero at the centre of the bit.Thus, by not drilling the centre of the wells, the bits have gainedefficiency. However, this type of bit produces a core in its centre,which must be broken up or evacuated.

Various types of bits which do not drill the centre of wells are known.

In particular, a bit is known from document U.S. Pat. No. 2,931,630comprising an array in the surface of which a plurality of diamonds ismounted. This bit comprises moreover a cavity for receiving a core, thecore being periodically broken off and evacuated by moving towards theoutside and above the bit. The array in which the plurality of diamondsis mounted makes it possible to drill through hard and very hard rocks.However, if the bit encounters soft rock, the spaces situated betweenthe diamonds become clogged and the tool can no longer drill. Now,during the drilling of wells of great depth, different types ofgeological formations are passed through by the bit and it is highlyprobable that soft rock will be encountered. This type of tool istherefore not suitable for drilling wells of great depth.

Bits provided with a chamber for crushing the core formed are known fromthe documents FR-A-2 141 510 and FR-A-2 197 325. However, if the bitencounters soft rock, the crushing chamber becomes clogged. The bitprovided with a crushing chamber must then be brought out so that thebit can be cleaned, which leads to a significant loss of time.

From the document BE-A-1 014 561 a bit is also known, provided with ameans or a device suitable for destroying the core in a progressive orcontinuous fashion or periodically, this means or this device beingsituated in the central zone of the body of the bit. In one embodiment,the means for destroying the core is the side wall of the central zoneof the body of the bit. The core is then broken periodically under theeffect of transmitted mechanical vibrations. However, if the bitencounters soft rock, the central zone of the bit becomes clogged. Itmust then be brought out to be cleaned, which leads to a considerableloss of time.

SUMMARY

The purpose of the invention is therefore to propose a bit which makesit possible to rapidly drill wells of great depth in all types of rockswithout the risk of clogging.

This purpose is achieved by a bit for drilling wells comprising:

-   -   a front face    -   a plurality of radial blades provided with cutting elements, the        blades being distributed around the front face,    -   a space for forming a core, the space being situated at the        centre of the front face, and    -   a cavity for evacuating the core towards the periphery of the        bit, the cavity being situated between two adjacent blades.

According to another feature, the cavity is delimited by two lateralsurfaces and a clearance surface, the clearance surface being set backwith respect to the front face.

According to another feature, the clearance surface is suitable forevacuating the core simultaneously towards the periphery and towards therear of the bit.

According to another feature, the bit further comprises a device forbreaking up the core.

According to another feature, the breaking device is situated in thecavity close to the centre of the bit.

According to another feature, the breaking device is a tip made of anabrasion-resistant material.

According to another feature, the tip is inclined with respect to theaxis of the bit.

According to another feature, the dimensions of the cavity are suitablefor forming cylindrical cores the length of which is equal to at leasttwice their diameter.

According to another feature, the bit further comprises drilling mudfeed channels, each of the channels opening onto the front face.

According to another feature, one of the channels opens into the cavityand is suitable for facilitating the evacuation of the core towards theperiphery of the bit.

According to another feature, the bit further comprises an element forradially cutting the core.

Another purpose of the invention is to provide a method for drillingwells using the bit described above, comprising the steps consisting offorming a core at the centre of the bit; evacuating the core up throughthe drilling well to the ground surface; recovering the core.

According to another feature, the drilling method further comprises astep of analyzing the petrophysical properties of the core.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will becomeapparent on reading the following detailed description of theembodiments of the invention, given by way of example only and withreference to the drawings, which show:

FIG. 1, a perspective view of a bit (PDC tool) according to theinvention,

FIG. 2, a longitudinal cross-section view of a bit (impregnated tool)according to the invention,

FIG. 3, a front view of a bit (PDC tool) according to the invention,

FIG. 4, a side view of a cutting element,

FIG. 5, a longitudinal cross-section view of the front face of the bit.

DETAILED DESCRIPTION OF THE INVENTION

A bit according to the invention comprises a front face. The bit alsocomprises a plurality of radial blades provided with cutting elements,the blades being distributed around the front face. The blades, providedwith cutting elements, make it possible to drill, by shearing, in alltypes of rocks, thus avoiding clogging of the blades. A space situatedat the centre of the front face allows the formation of a core at thecentre of the bit. The formation of a core at the centre of the bitmakes rapid drilling possible. A cavity situated between two adjacentblades of the bit allows the core to be evacuated towards the peripheryof the bit. Thus, the evacuation of the core prevents any clogginginside the bit. The bit according to the invention therefore allows therapid drilling of wells of great depth in all types of rocks without therisk of clogging.

Identical reference numbers in the different figures represent identicalor similar elements.

FIG. 1 represents a perspective view of a bit according to theinvention. The bit comprises a body 12 having a rotational symmetryabout an axis 13. The bit is suitable for mounting on a drill pipestring and being rotationally driven by different types of motor, on thesurface or bottom, for example a motor with a spiral shaft (for exampleof Moineau type) or a turbine.

The front of the bit is defined as the part of the bit which isorientated towards the bottom of the well and the rear of the bit as thepart of the bit which is orientated towards the outside of the well,i.e. in the case of vertical drilling, the earth's surface.

Moreover, the inside of the bit is defined as the part of the bitsituated close to the axis 13 and the outside of the bit as the part ofthe bit situated close to the periphery of the bit.

The body 12 of the bit comprises a front face 1, which is preferablyrounded so as to facilitate the penetration of the bit into the rocks aswell as to provide the tool with satisfactory stability. The front face1 is provided with a plurality of blades 2, for example 4, 6 or 8blades, or even many more, for example 36. The harder the rocks to bedrilled, the higher the number of blades.

The blades 2 are arranged in a substantially radial fashion, as can beseen in particular in FIG. 3. The blades extend along the outside wallof the body 12. The blades 2 project with respect to the front face 1and to the outside wall of the body 12. Each blade 2 comprises aplurality of cutting elements 3 arranged alongside each other along theblade. The cutting element of a blade which is closest to the centre ofthe tool is called the inside cutting element of a blade. And thecutting element of a blade which is closest to the periphery of the toolis called the outside cutting element. Each cutting element 3 has asubstantially cylindrical shape. The cutting elements 3 are mounted inthe blades 2.

Each cutting element 3 is composed of material based on various metalsincluding, for example, tungsten carbide (WC).

In a first embodiment, the metal-based material, with or withouttungsten carbide, is impregnated with synthetic diamond, or even naturaldiamond, grains of varying sizes, ranging for example from 0.2 mm to 2mm. A tool provided with cutting elements according to this firstembodiment is called an “impregnated tool”.

In a second embodiment, illustrated in particular in FIG. 4, a layer ofpolycrystalline diamond compact, PDC, 32 is situated on the face of astud 31 made of case-hardened tungsten carbide. This layer of PDCcomprises a small quantity of metal so as to ensure itsshock-resistance. A tool provided with cutting elements according tothis second embodiment is called a “PDC tool”.

The cutting elements of the impregnated tools and PDC tools are veryhard and thus make it possible to drill rocks of variable hardness andin particular very hard rocks. The cutting elements are suitable forbreaking rocks by shearing, which also makes them suitable for drillingin soft rocks.

The body 12 and the blades 2 of the bit are for example made of steel orinfiltrated WC. They are preferably made of steel as this material ismore resistant than infiltrated WC. Steel therefore allows more variedgeometries of the bit, making it easier to adapt to the ground to bedrilled.

Moreover, the blades 2 are arranged on an external annular crown of thefront face 1. A space 4 is thus situated approximately at the centre ofthe front face 1. This space 4 is situated approximately at theintersection of the planes of the blades. This space is delimited by theinside cutting elements of each blade. When the bit is driven into therock while rotating, a substantially cylindrical core 10 is formed inthis space 4.

Moreover, the bit comprises an evacuation cavity 5 situated between twoadjacent blades 2. This evacuation cavity 5 is suitable for evacuatingthe core towards the periphery of the tool. The evacuation cavity 5 isdelimited by two lateral surfaces 6 and a clearance surface 7. Thelateral surfaces are substantially parallel, even merged, with thelateral surfaces of the two blades adjacent to the cavity. The anglebetween the adjacent blades, between which the cavity is formed, is forexample comprised between 45° and 90°. This angle is a function of thediameter of the tool and that of the core formed. The clearance surface7 is set back with respect to the front face 1. The clearance surfacecan be seen particularly well in FIG. 2. The clearance surface 7 extendsfrom the space 4 to the periphery of the bit. The base of the space 4 issituated in the cavity 5. The clearance surface 7 rises towards the rearof the bit, and extends along the tool guard. Thus, the clearancesurface 7 allows the core 10 to be guided simultaneously towards theperiphery of the bit (which is facilitated by centrifugal force) andtowards the rear of the bit (which is facilitated by the forwardmovement of the tool and by the drilling mud) in order to evacuate itinto the well. Once the core is evacuated from the cavity 5, it riseswith the drilling mud to the earth's surface.

Moreover, the bit comprises a breaking device 11, suitable for causingthe core to break by shearing. The breaking device 11 is situated on theclearance surface 7 of the cavity 5, close to the centre of the bit. Thebreaking device 11 is for example fixed onto this clearance surface 7,for example by crimping. During the formation of the core, thelength/diameter ratio of the core increases. The longer the corebecomes, the more the latter weakens. Therefore a small lateral pressureis sufficient to cause it to break. The breaking device 11 can thereforebe any device which is capable of producing such a lateral pressure. Thebreaking of the core occurs when the core attains a length which isdetermined by the depth of the space 4 (namely the distance between thefront of the blades 2 and the evacuation cavity 5 at the centre of thebit) and the positioning of the breaking device 11 with respect to theaxis 13 of the bit.

The breaking device is for example made of an abrasion-resistantmaterial, for example a metal-based material, with or without tungstencarbide, diamond impregnated, or of PDC, or also of ceramic or of acarbide-based material. The breaking device 11 is for example in theform of a tip. The tip is arranged according to an axis which isinclined with respect to the axis 13 of the bit, as can be seen inparticular in FIG. 2. The angle between the plane of the tip and theaxis of the bit is for example comprised between 10° and 15°.

The dimensions of the core 10 are limited by the geometry of the bit,and in particular by the geometry of the space 4 and the cavity 5.

Moreover, the bit comprises channels 8, 9, which can be seen inparticular in FIG. 3, which are suitable for conveying drilling mud, thedrilling mud making it possible to cool the bit down and to raise therock cuttings up through the well to the earth's surface.

The drilling mud also makes it possible to raise the cores formed in thebit up to the earth's surface.

Economic investments in the oil industry mean that it must be possibleto realistically describe the geological structure of wells andreservoirs, for example by having access to the petrophysical properties(porosity, permeability, etc.) of the rocks constituting the wells andthe reservoirs. These properties are not uniform in any reservoir, butdepend on the geological structures which constitute it. This results inreservoir heterogeneity. Knowledge of the reservoir involves thedetermination of such heterogeneities. The characterization of wells andreservoirs makes it possible to offer assistance in deciding on theevolution of development of the deposit and more generally, it makes itpossible to provide assistance as regards the exploitation of thedeposit and the drilling of wasteland.

In the case of wells of great depth (typically 5-6 km), the pressuresand temperatures at the bottom of wells are such that it is impossibleto carry out standard characterizations such as standard loggings orcorings. In fact, the electronics used for loggings is not resistant tohigh pressures (7800 bar or more) and high temperatures (150° C. ormore). In addition, the standard coring is very restricting as itassumes that the core obtained is raised to the surface every 10 to 40 mof drilling.

Also, it is particularly useful to be able to continuously raise to thesurface the cores formed by the bit according to the invention in orderto be able to carry out the characterizations of the wells at thesurface. It is also advantageous for the cores to be of a length whichis sufficiently great to be able to extract a maximum amount ofinformation on the geological structure of the well.

In order that the core 10 is raised to the surface as intact aspossible, it is necessary that the breaking device 11 does not crush it,but shears it.

It has been observed that the core is sheared and not crushed when thelength/diameter ratio is at minimum equal to 2. The dimensions of theevacuation cavity 5 must therefore be at least equal to the greatestdimension of the core, i.e. its length.

The cores obtained by the bit according to the invention have a lengthof the order of 10 to 100 mm.

In the case where the rocks to be drilled are hard, the bit comprises ahigher number of blades than in the case where the rocks to be drilledare softer. The outside diameter of the bit is for example 21.59 cm(8.5″) for a bit with 8 blades, 15.24 cm (6″) for a bit with 6 bladesand 66.04 cm (26″) for a tool with 36 blades.

For a bit with 8 blades with a diameter of 21.59 cm, cores of length 35mm and diameter 15 mm have been obtained.

For a bit with 6 blades with a diameter of 15.24 cm, cores of length 30mm and diameter 10 mm have been obtained.

The maximum diameter that can be envisaged for a core is approximatelyequal to one-third of the outside diameter of the bit. In order to beable to exploit the cores satisfactorily, it is desirable for thediameter of the core to be at minimum equal to 5 mm.

Moreover, the presence of a core at the centre of the bit has astabilizing effect on the bit. The greater the diameter of the cores,the more stable the bit during drilling.

Moreover, the cylindrical shape of the core makes it possible to providea directional reference, the axis of the core corresponding to the axisof the well drilled.

The core 10 is sheared by the breaking device 11 of the bit, thenevacuated in cavity 5 towards the periphery of the bit then raised upthrough the well to the earth's surface with the drilling mud.

The bit comprises for example a number of channels 8, 9 supplyingdrilling mud, equal to the number of blades. The channels 8, 9 open outonto the front face 1 of the bit.

One of the channels 9 opens into the cavity 5 close to the centre of thebit and the breaking device 11. This channel 9 facilitates theevacuation of the core in the cavity along the clearance surface 7towards the periphery of the bit. During its evacuation via the cavity,the core is thus steeped in the drilling mud. This reduces the risk ofthe core knocking against the lateral walls 6 or the clearance surfaceof the cavity is reduced. The core is therefore less likely to break up.

The orifices of the other channels 8 are arranged substantially aroundan axial crown, as can be seen in particular in FIG. 3.

Another embodiment is illustrated in particular in FIG. 5, whichrepresents a longitudinal cross-section view of the front face of thebit. FIG. 5 shows the cutting elements 3 mounted on a blade. Inside thebit, along the axis 13, a core 10 is represented in the process of beingcreated in the space 4. According to FIG. 5, the dimensions of the space4 are increased. This allows higher drilling speeds to be achieved.Moreover, the bit comprises a component 14 for radially cutting thecore. The component 14 can be situated at the centre of the bit. Thecomponent can be arranged laterally with respect to the space 4. Thiscomponent is presented, for example, according to the cutting element 3described above. The component 14 is for example mounted in the bit, therotation of the bit making it possible to reduce the diameter of thecore by cutting the core with the component 14. The reduction in thediameter of the core makes it possible not only to raise the core to thesurface more easily, but also to raise the core without damaging it.Therefore, it is possible to increase the size of the space 4, and thusguarantee rapid drilling, while keeping a core intact. By way ofexample, it is possible to use a bit with 6 blades (6″) having a space 4with a 20 mm diameter. Starting with a core diameter of 20 mm, thecomponent 14 makes it possible to obtain a core of 8 mm.

The invention also relates to a method for drilling wells using the bitaccording to the invention. The method comprises the steps consistingof:

-   -   forming a core at the centre of the bit,    -   evacuating the core up through the well to the ground surface,    -   recovering the core, for example in a sieve.

The drilling method also comprises the step consisting of analyzing thepetrophysical properties of the core.

The drilling method also comprises the step consisting of analyzing themechanical properties of the core.

What is claimed is:
 1. A bit for drilling wells, comprising: a frontface; a plurality of radial blades provided with cutting elements, theblades being distributed around the front face; a space for forming acore, the space being situated at the center of the front face; and acavity for evacuating the core towards a periphery of the bit, thecavity being situated between two adjacent of the blades and preventingany clogging inside the bit.
 2. A bit according to claim 1, wherein theradial blades are arranged on an external annular crown of the frontface.
 3. A bit according to claim 1, wherein a majority of the cavity issituated between two adjacent blades, wherein the cavity is delimited bytwo lateral surfaces and a clearance surface, and wherein the clearancesurface is set back with respect to the front face.
 4. A bit accordingto claim 1, further comprising drilling mud feed channels, wherein eachof the channels opens onto the front face, and wherein one of thechannels opens into the cavity and is suitable for facilitating theevacuation of the core towards the periphery of the bit.
 5. A bitaccording to claim 1, wherein the clearance surface is suitable forevacuating the core simultaneously towards the periphery and towards therear of the bit.
 6. A bit according to claim 1, further comprising adevice for breaking up the core.
 7. A bit according to claim 6, whereinthe breaking device is situated in the cavity close to the center of thebit.
 8. A bit according to claim 7, wherein the breaking device is a tipmade of an abrasion-resistant material.
 9. A bit according to claim 8,wherein the tip is inclined with respect to the axis of the bit.
 10. Abit according to claim 1, wherein the dimensions of the cavity aresuitable for forming cylindrical cores the length of which is equal toat least twice their diameter.
 11. A bit according to claim 1, furthercomprising an element for radially cutting the core.
 12. A method fordrilling wells, comprising: providing a bit for drilling wells,comprising: a front face; a plurality of radial blades provided withcutting elements, the blades being distributed around the front face; aspace for forming a core, the space being situated at the center of thefront face; and a cavity for evacuating the core towards a periphery ofthe bit, the cavity being situated between two adjacent of the bladesand preventing any clogging inside the bit; forming a core at the centerof the bit; evacuating the core up through the drilling well to theground surface; and recovering the core.
 13. A method for drilling wellsaccording to claim 12, further comprising analyzing the petrophysicalproperties of the core.